Isolation and characterization of the enzymes from Crithidia fasciculata leading from orotate to UMP and CMP. A study of the enzymes of Crithidia and Tetrahymena which interconvert purines. A detailed study of the purine and pyrimidine (and their ribonucleotides) transport systems in Crithidia and Tetrahymena, and their relation to the methylxanthines. BIBLIOGRAPHIC REFERENCES: Kidder, G.W., V.C. Dewey and L.L. Nolan, Enzymes of the orotate biosynthetic pathway in Crithidia fasciculata. Can. J. Biochem., (In Press, Feb. 1976 issue). Kidder, G.W. and L.L. Nolan, Enzymatic synthesis of labeled carbamylaspartic acid. Biochem Biophys. Res. Comm., 65, 420 (1975).